Ryu was historically a feudal institution created by the various clans, for the purpose of teaching martial arts to the samurai of their clan. Ryu (流) translates as "school" or "style" (to be understood as a school of thought); just think of how many martial arts in general adopt the suffix Ryu.   

As with many Japanese words, these have multiple possible translations, 
in fact Ryu is also translated as "flow of a river". 

This concept however, contrary to what it seems, does not differ from the translation of "school/style", indeed these two translations go hand in hand, since it is understood that the knowledge of a martial art, within a school, is passed down from generation to generation and flows until reaching the present day. 

The word Ryu, used mainly as a suffix, means style, type, form, way, system, school; here as Ryu-Ha 流 a school, a school of thought. Ryu in Japanese martial arts are classified and codified in Ryu-Ha. 

Usually, a specific style has its own curriculum, ranks and licensing system. 

There is no universal licensing or ranking system in all Ryu-Ha. A person with a high rank in one style does not necessarily correspond to a high level in another style.

SOKE 宗家

Head of a Japanese Ko Ryu. The Soke can formalize schools that follow a traditional path of study by appointing a Soshi Soke. The Soke has the authority to issue Menkyo licenses to those who have mastered his style. The Soke can appoint his Waka Sensei (Young Master) who will replace him after his voluntary or forced retirement in the event of his death. The Waka Sensei can legitimately use the title of Soke. 

SOSHI SOKE 創始宗家

 It is a Japanese term that means "Presenter of an idea, method or style". 
This classification formalizes the School of Soshi Soke as Ryu Ha. This can only be recognized by a Japanese Ko Ryu with the signature of his Soke. The Soshi Soke can nominate his Waka Sensei (Young Master) who will replace him after his voluntary or forced retirement in the event of his death. 
The Waka Sensei will be able to fully claim the title of Soshi Soke.


GAI SOKE 外宗家 

It is a Japanese term to define a foreign (non-Japanese) Headmaster. 
The IBA, with the favorable opinion of the Presidency Office, will formalize the requests of the Headmasters of Japanese Styles or Methods of a discipline, after having verified the documentation presented by them and having received the approval of the "FISECAM" Presidency Office. 

The requesting Master can be accredited as a "Gai Soke" Headmaster only if he has a Kodansha diploma position (at least 5th Dan) and is not less than 40 years old. 

The Gai Soke can nominate his Waka Sensei (Young Master) who will replace him 
after his voluntary or forced retirement in the event of death. 
The Waka Sensei will be able to fully boast the title of Gai Soke.